It is known that the main difficulties in canning tuna are obtaining cans of constant weight, so as to avoid production waste, and presenting the consumer with a good-looking product when the can is opened, since this determines the product value to a great extent. Such difficulties are not easy to overcome due to the intrinsic nature of tuna, which is a food product showing ample variations in compactness, density and shape from batch to batch where not even from loin to loin.
Moreover, the manufacturer tries to obtain the maximum quantity of finished product from the raw material, which should therefore be treated so as to avoid as much as possible crumbling and loss of liquids that lead to a decrease in weight of the raw material to be canned. Clearly, all of the above should be achieved through a machine that guarantees an adequate productivity, since machines and methods that are too slow result in excessive costs.
The main phases of the canning process are therefore the separation from the bulk of fed product of a tuna cake having a suitable weight, neither too low to risk obtaining an underweight can nor too high to reduce the yield of the raw material, and the shaping thereof into a shape suitable for the introduction into a can, typically a round cylindrical shape. In the following, specific reference will be made to the canning into conventional round cans, yet it is clear that what is being said is also applicable to the canning into cans having other shapes such as oval, rectangular with rounded corners and the like, as well as into jars or other containers.
Prior art machines and methods can be substantially divided in two categories depending on the sequence of the above-mentioned main phases, i.e. first dosing and then shaping or vice versa. In practice, in a first type of machine the product is shaped while being fed to the dosing chamber and the cake that is cut from the bulk of product already has a shape suitable for canning, whereas in a second type of machine a cake of suitable weight and generally quadrangular shape is cut from the bulk of product and subsequently shaped for the introduction into the can.
A recent example of a machine of the first type can be found in WO 2004/103820 that discloses a machine for obtaining simultaneously two conventional round cans, comprising a forming mouth, with a rectangular inlet and a binoculars-shaped outlet, which is crossed by a vertical knife that reciprocates perpendicularly to the feed direction to divide the tuna loin in two portions. Said mouth connects the conveyor belt tuna feeder to two dosing chambers formed in a rotor that rotates in a plane perpendicular to said feeder to take the two dosing chambers to a second station where the round cakes are transferred into the cans. This type of machine has several drawbacks resulting from the high push on the tuna required to go from the rectangular inlet portion of the mouth to the cornerless outlet portion.
A first drawback is the damage to the outer surface of the tuna that scrapes with high friction along the inner walls of the mouth to follow the great variation in shape of the cross-section; such a friction also causes a compression of the peripheral fibers of the tuna which therefore results having a non-uniform density when leaving the mouth. This compression also causes the further drawback of a “squeezing” of the tuna with loss of liquids and crumbles, which not only reduce the yield of the raw material but can also leak through the interstices of the machine causing the mechanisms to get soiled and clogged.
Still another drawback caused by such a friction is the fact that the central fibers of the tuna are more unimpeded in advancing with respect to the peripheral fibers whereby the cake that is obtained after the cut tends to be convex. This may cause problems in the steps following the canning since the central portion of the can, being higher, may get in contact with the can lid and therefore burn during the sterilization process or it may not be sufficiently covered by the control liquid (oil or other).
Finally, it should be noted that this canning method is even more sensible to the already high intrinsic variability of tuna, since the push of the conveyor belts on the tuna must be continuously adjusted and is affected by the flow of the bulk of fed product and by possible irregularities or pauses in the infeed. This also affects the precision in determining the cake weight, despite the presence of load cells that control the operation of the conveyor belts depending on the push exerted by the tuna on bottom plugs that close the dosing chambers.
The most common example of the second type of machine has remained practically unchanged in the last three decades and is described in U.S. Pat. No. 4,116,600: the tuna is cut in an approximate amount by a knife located at the end of the conveyor belt feeder, then pushed perpendicularly by a ram into a metering pocket with a semicircular concave bottom where a second knife closes the pocket and defines the exact amount. This metering pocket consists of two adjacent peripheral pockets formed in two rotating turrets between which there is arranged a third knife that divides the thus formed tuna cake in two cakes, and each turret then rotates independently towards a second station where the shaping is completed by a relevant radial plunger shaped with a concave semicircular contact surface prior to moving the cake to a third station where the transfer into the can takes place.
Although this type of machine does not subject the tuna to the high friction of a forming mouth as in the first type of machine, nonetheless it also has various drawbacks of a different kind.
In the first place, the product dosing is achieved by filling the metering pocket by means of the perpendicular ram that must compress the tuna with a pressure as uniform as possible in order to obtain a density and therefore a cake weight which is constant. However, as discussed above, the intrinsic nature of tuna and the irregularities in shape, infeed and flow make it difficult to achieve a constant weight, in particular since there are no load cells or other systems that provide a feedback to the feeder. On the other hand, increasing the ram force in order to reduce the effect of such irregularities leads to the “squeezing” of the tuna with increased damage to the product and a lower yield.
Secondarily, although the tuna is not forced through a forming mouth yet it undergoes three cuts along different surfaces and two displacements before obtaining the final shape: a first displacement by the ram scraping perpendicularly to the conveyor belt to enter the metering pocket, and a second displacement in the turret scraping against the inner surface of the machine casing between the first and second station. This still implies various frictions with subsequent losses of liquid and risks of crumbling, in addition to a certain degree of complexity of the machine that also has a low productivity exactly due to the several movements required to perform this canning method. Moreover, the rotating speed of the turrets can not be too high in order to prevent the centrifugal force from increasing the friction of the tuna against the casing during the rotation.
The subsequent improvements to this machine disclosed in the patent publications U.S. Pat. No. 5,887,413 and WO 2008/109084 respectively relate to the possibility of changing the cake thickness by means of adjustable end plates and the possibility of always having the surface of the last cut facing the can lid thanks to opposite knock-out plungers, yet they do not overcome any of the above-mentioned drawbacks.
The same drawbacks, even to a higher degree, are present in the machine disclosed in EP 1448445 that performs a similar canning method but it provides the division of the cake in the metering pocket by pushing the tuna against a fixed blade and a subsequent sub-division in a second chamber by pushing it against a second fixed blade prior to shaping. It is obvious that the higher number of displacements and the use of fixed blades increase the friction, the losses and the damage to the product.